DESCRIPTION: The central goal of this research is to advance understanding of how infants acquire motor skill. The hallmarks of skill are fluency and flexibility. Previous research on motor skill acquisition in infants has focused primarily on changes in fluency, indexed by differences in rapidity, smoothness, and accuracy of performance at various ages. This research, in contrast, addresses three aspects of flexibility: adapting ongoing movements to changing circumstances, deploying a variety of different strategies to arrive at a solution, and transfer of old movement strategies to new motor problems. Understanding flexibility requires a heightened focus on learning, especially the processes involved in appropriate accommodation to novel tasks. The proposed experiments present 8- to 16-month-old crawling and walking infants with various locomotor problems (descending slopes and stairs, crossing gaps and bridges, etc.) to examine flexibility of exploratory movements and locomotor strategies. The studies employ a microgenetic approach, providing a fine-grained, quantitative description of movements, and a complementary developmental approach, relating responses in the lab tasks to ongoing changes in infants' bodies, skills, and everyday experience. The first three experiments focus on adaptation of ongoing exploratory procedures and locomotor strategies at different developmental levels to examine the informational basis for infants' strategy choices. Experiments 1 and 2 assess how "expert" walking infants adapt to experimental manipulations of surface friction or body proportions. Experiment 3 tracks developmental change in exploratory procedures and strategy choices in novice and experienced crawling, "cruising," and walking infants on surfaces varying in extent (width of a gap or a bridge). The second three experiments focus on variety of strategy choices and transfer of locomotor strategies from one problem to another. Experiment 4 tests effects of concentrated practice on learning to modify an existing locomotor strategy and apply it appropriately. Experiment 5 examines whether availability of manual supports promotes use of a "tool" to augment locomotor skill. Experiment 6 tests effects of instruction and training problem on transfer to new descent problems and variety of strategies deployed. Of special interest are the micro- processes of strategy acquisition. The emphasis on flexibility shifts the focus of this research on motor development toward central issues in psychology regarding perceptual learning, strategy acquisition, and strategy choice. Together, the experiments will advance understanding of how newly mobile infants learn to cope with an ever expanding environment. In addition, the results may have practical applications for promoting new skills in normally developing infants and in children with motor impairments.